This invention relates to the application of hot melt adhesive compositions and to an apparatus for melting and application of such systems.
Hot melt thermoplastic adhesive resins are generally supplied in granular or pellet form and are heated to form a liquid melt for application to substrates. When large volumes of hot melt adhesive are required, the resin has usually been melted in a large tank reservoir and supplied to the applicator by means of a gear pump. Such equipment requires long start-up times to melt the adhesive and suffers from the disadvantages of excessive degradation of resin exposed to high temperatures for long periods of time in the melt tank and difficulty of cleaning of the equipment to accommodate change in adhesive composition. Several modifications of such systems have been made to reduce the exposure of the adhesive to high temperatures. Such modifications include localized heating of a block of adhesive, and controlled addition of granules of adhesive to a hot platten or hot grid. In such systems the melt is immediately pumped to the applicator. However equipment which incorporates any such modifications also possesses several deficiencies. For example such equipment is not efficient for application of large volumes of adhesive, it is difficult to maintain at high temperatures, it tends to hold up some resin which degrades and causes variable adhesive performance and it is very troublesome to clean.
Small scale extruders in hand-held applicators have been used for supplying heated caulking compounds, mastics or sealants to a work piece. Such hand-held applicators are fed with plasticized elastomeric compositions in rope form and apply the elastomeric compositions as a ribbon which emerges directly from the extruder exit. The applicators are very limited in capacity and are not designed to accept granules of thermoplastic polymers since the extruder feed zone is operated at a higher temperature than the melting zone. In an apparatus containing a feed screw capable of dispensing large volumes of mastic material, the mastic material is heated in the hopper to soften it before it flows down to the feed screw. Such an apparatus has all the deficiencies of the tank and gear pump dispensers discussed hereinabove.
A small scale apparatus for filling dispensing guns with hot melt thermoplastic adhesive has been disclosed. The thermoplastic in granular form is fed from a hopper to an extruder barrel and the molten adhesive from the extruder is injected into the barrel of the dispensing gun through the gun nozzle which is forcibly held against the extruder outlet by means of a spring loaded socket. When the gun barrel is full, the extruder stops and the gun is removed from the socket. Only small amounts of adhesive are thus available for application and must be applied immediately since the gun has no means for heating. Also overflow from the extruder tends to make the necessary repeated filling of the gun a messy operation and causes frequent interruptions to allow the extruder exit and the gun nozzle to be cleaned.
An apparatus has now been devised which overcomes these several disadvantages and allows hot melt adhesive to be delivered at high rates intermittently or for long periods of time. The apparatus comprises:
(a) a single screw, extruder equipped with means to rotate the screw and with means to start and stop the screw rotating means, and having a first zone for receiving solid thermoplastic, a second zone for melting the thermoplastic, and a third zone for metering the discharge of the molten thermoplastic from the extruder;
(b) a reservoir containing the thermoplastic in the form of solid granules, the reservoir being attached to the barrel of the extruder at the receiving zone and having a throat communicating with the extruder barrel, through which the solid granules are fed to the extruder;
(c) cooling means at the receiving zone of the barrel of the extruder to allow the thermoplastic therein to remain in granular form until it is passed to the melting zone of the extruder;
(d) heating means in the barrel of the extruder to progressively heat and melt the thermoplastic in the melting zone and maintain it molten in the metering zone; and
(e) a heated manifold to receive the molten thermoplastic from the discharge end of the extruder, to maintain it in the molten state and to distribute it to at least one applicator; said at least one applicator comprising a housing, a barrel to receive the molten thermoplastic from the manifold, the barrel terminating in a nozzle from which the molten thermoplastic can be discharged, heating elements and heat sensors attached to the barrel to allow the thermoplastic to be maintained in the molten state, means in the applicator barrel to allow the molten thermoplastic to flow through the barrel and be discharged from the nozzle, and means to actuate and de-actuate the flow-allowing means; wherein the means to actuate the flow-allowing means in the applicator barrel also actuates the starting means of the extruder and the means to de-actuate the flow-allowing means in the applicator barrel also actuates the stopping-means for the screw-rotating means.
Preferably the apparatus is equipped with a pressure overload sensing means which actuates the means for stopping the screw-rotating means when the pressure reaches a set overload value.
The advantages provided by this apparatus include the capability of delivering melt over a broad range of delivery rates, the ease of varying and regulating delivery rates, the short residence time of the thermoplastic in the heated zones of the apparatus, the ease of achieving and controlling high temperatures in the melt, the ease of cleaning the apparatus and changing thermoplastics, the option of using robotic control of the discharge of the melt from the applicator, the option of attaching a plurality of hoses and applicators, and the option of attaching an accumulator to the manifold to provide a reservoir for the melt.
The apparatus with an accumulator attached to the manifold provides a preferred embodiment of the invention. The accumulator comprises a piston cylinder to receive molten thermoplastic from the manifold, a piston to deliver molten thermoplastic from the cylinder to the manifold, a piston ram, a piston ram control, means to actuate the piston ram control, means to de-actuate the extruder screw-rotating means, and heating means attached to the piston cylinder to maintain the thermoplastic in the piston cylinder at the desired temperature, wherein during the applicator discharge mode the piston drive control is actuated to cause the piston to deliver molten thermoplastic from the piston cylinder into the manifold, wherein during the applicator closed mode, the piston drive control, and the screw-rotating means of the extruder are actuated to allow the extruder to deliver molten thermoplastic via the manifold to the piston chamber; and wherein when the piston cylinder is full of molten thermoplastic during the applicator closed mode, the extruder screw-rotating means is de-actuated. The accumulator provides the advantage during extended periods of intermittent operation of the applicator of allowing the extruder to run continuously and the advantage of providing an applicator discharge rate which can be appreciably greater than the extruder output.